(1) Field of the Invention
The present invention relates to a catalyst for the hydrotreatment of heavy oils and a method for the preparation of the catalyst. More specifically, it relates to a hydrotreating catalyst for removing sulfur, nitrogen and metals such as nickel and vanadium contained in the heavy oils, and to a method for the preparation of the catalyst.
(2) Description of the Prior Art
As a worldwide petroleum situation in recent years, there is a tendency of manufacturing a larger amount of lighter oils, and in consequence, the treatment of heavier oil fractions is present as a problem. Particularly in residual oil from which the useful lighter oil fractions have been distilled out, sulfur, nitrogen and metals are concentrated. In other words, the concentration of these undesirable components is higher in the residual oil. There are a few processes to treat heavy oils to get the lighter fractions, for example, fluid catalytic cracking and hydrocracking. In the practice of such a process, sulfur, nitrogen and metals contained in the heavier oils cause air pollution, deteriorate the quality of products, or are a catalyst poison.
For this reason, a process for removing sulfur, nitrogen and metal by hydrotreating the heavier oils is getting more important. This hydrotreatment is a process in which the material oil is catalytically treated under a hydrogen pressure in order to remove the sulfur and the nitrogen therefrom by converting them into hydrogen sulfide and ammonia, or to remove the metal content therefrom by depositing it on the catalyst. Heretofore, an indirect desulfurization has been particularly utilized to mainly treat vacuum gas oils, and thus the metal content in the material oil has been small. However, along with the worldwide increase in the manufacture of the lighter oils from crude oil in recent years, metals such as nickel, vanadium and the like have increased in the residual oil, and in consequence, the residual oil has often been treated by a direct desulfurization.
A problem of such a process is that pores in the catalyst are plugged wtih the metal in the oils and coke, so that the activity of the catalyst will be poor and the operation of the apparatus will be impeded. Accordingly, in order to keep the stable operation of the apparatus, it is necessary to develop a catalyst having a long life which can effectively remove the metals contained in the material heavier oils and which can maintain its activity for a long period.
Many of the catalysts for the hydrotreatment of the heavy oils are in the form of particles or extrudates each having a diameter of several millimeters to 1 cm. In general, the catalysts are made by a solution impregnation method, which comprises dissolving a catalytically active component in a solvent, immersing a carrier into the resultant solution, removing the carrier therefrom after a predetermined period, drying and then calcining.
When the active metal is supported in the carrier in accordance with the above method just described, the concentration of the active metal is uniform in the carrier or is high in the vicinity of the outside surface of the carrier.
When the hydrotreatment of the heavy oils is carried out by the use of the thus prepared catalyst, metals in the heavy oils and deposited in the vicinity of the outside surface of the catalyst, so that pores in the catalyst are plugged therewith, with the result that active sites in the catalyst are not utilized sufficiently.
As means to eliminate this disadvantage, there is a method (Japanese Patent Provisional Publication No. 132945/1984) in which the concentration of the active metal is lowered to inhibit the deposition of molecules containing the metals in the heavy oils in the vicinity of the outside surface of the carrier and to thereby allow the molecules to diffuse into the interior of the catalyst, whereby the active point in the catalyst is used effectively.
Further, another method (U.S. Pat. No. 4,225,421) is also present in which the diameter of each pore is enlarged or a bimodal carrier is employed for the purpose of allowing the molecules containing the metals to diffuse to the interior of the catalyst.
However, in the former method, the overall activity of the catalyst is low, and in the latter method, the surface area of the catalyst is small and each pore volume is large, which facts require a great deal of the active metal to be supported.